(a) Field of the Invention
The subject invention relates to a unique airborne survey bird with a geophysical survey system for natural resource exploration of oil and gas, mineral deposits and aquifers and using natural electromagnetic fields as an energy source.
(b) Discussion of the Prior Art
Heretofore, airborne electromagnetic systems have been in use for natural resource exploration from about 1950 onwards. These systems depend mainly upon the measurement of the magnetic and conductive properties of the underlying ground. Airborne magnetic survey systems, that employ magnetometers with advanced stages of development, provide very satisfactory results. However, airborne conductivity measurements of the underlying terrain made with airborne electromagnetic systems that currently exist, leave a great deal of room for improvement. These systems typically operate at a minimum terrain clearance with respect to safety and employ electromagnetic transmitters operating in the frequency range from about 20 Hz. to 50 kHz with limited ground penetration. The secondary fields induced in the underlying ground by these transmissions are detected by receiving coils mounted in a tail boom on an aircraft or in an airborne survey bird towed behind the aircraft Either fixed wing aircraft or helicopters are used for these surveys. The response from the underlying ground is related to it""s conductivity and the depth of penetration of the transmitted fields. The latter is primarily a function of the frequency employed and the field strength of the electromagnetic field that is generated by the equipment. Typical maximum penetrations are in an order of up to 1000 ft.
The only exception to the above description was an airborne system known as xe2x80x9cAFMAGxe2x80x9d that was developed by S. H. Ward and others in the 1960""s. (S. H. Ward et al. AFMAG-Applications and Limitations. Geophysics, Vol. XXXI, No. 3 (June 1966), pp. 576-605.) This system utilized the natural electromagnetic fields generated by lightning events occurring in distant electrical storms. These storms can provide a source for electromagnetic energizing of the ground, primarily in the frequency range of 20 Hz. to 500 Hz. Useable frequencies down to about 3 Hz. exist but high quality receiving coils and coil anti-vibration mountings are required for the lower frequencies. These were apparently not available in the AFMAG system.
Although the AFMAG system showed some promise, it did not achieve sufficient commercial acceptance to survive for more than a short period. Amongst the various problems of the system was the absence of the sophisticated instrumentation and digital data acquisition and processing systems that were not available at that time. Also, very importantly, there was a lack of adequate technology for suppressing the prime sources of noise, such as angular vibration of the detection coils in the presence of a strong magnetic field in the earth. The latter is associated with a motor generator effect that can detect a millionth of a degree of angular vibration.
The AFMAG system was also restricted to the use of audio frequency fields and did not employ extremely low frequency and much more powerful natural magnetotelluric fields, as used in the present invention. Just as importantly, the AFMAG system as well as all other airborne electromagnetic systems, past or present, did not make use of the valuable data available in the electric field components of electromagnetic fields.
The present invention demonstrates that electric field data, as measured by methods that do not make contact with the ground, can be more important than the magnetic component of electromagnetic fields. Experience with the present invention has also shown that, for specific reasons, the measuring of the electric field data is particularly valuable at frequencies below 3 Hz. This type of information is completely missing in the old AFMAG system as well as current airborne electromagnetic systems. The field data lies in the range of frequencies from 0.01 Hz. to 3 Hz. and is used in the present invention for the airborne detection of an induced polarization phenomena Also, related response of dielectric interfacial polarization effects can be detected over aquifers and oil and gas fields. These low frequency polarization effects, which are particularly important in the electric fields, are discussed herein.
A still further very important factor in comparing the present invention with other systems, is that for certain specific reasons, as presented, the operation can function at unusually high terrain clearances of 1000 ft. to 2000 ft. All other airborne systems that use transmitters have to fly at clearances in the range of 300 to 500 ft., which adds to potential problems related to interferences from power lines and pipelines. These interferences badly degrade data quality and often in areas where discovery potential is the greatest. With the present invention operating at much greater clearances and at very low frequencies, these problems disappear for all practical purposes.
A primary object of the subject invention is to provide a unique airborne survey bird using a new airborne electromagnetic technique for deep exploration using natural magnetotelluric fields as an energy source and operating in a frequency range of 480 Hz. down to 0.01 Hz The new survey system provides both deep and shallow exploration information, such as induced polarization effects, which heretofore was not available using current airborne systems and ground geophysical systems.
Another object of the airborne survey bird is it can operate at much higher terrain clearances from 1000 to 2000 feet when compared to other airborne systems operating in a range of 300 to 500 feet above the ground surface.
Yet another object of the invention is provide an airborne survey system on the survey bird that can explore for oil and gas, mineral deposits and aquifers with penetration down to thousands of feet in the ground surface. Also, the survey system detects very important effects in three orthogonal components of an electric field, which are not observed in other airborne or ground geophysical exploration systems. Further, the airborne survey system can detect underground aquifers using airborne methods for identifying dielectric double layer interfacial polarization effects associated with an abnormally high dielectric constant of water in contact with earth materials having a dielectric constant and conductivity that is fractional when compared with water.
Still further, the invention can be employed with a fixed network of stations on the ground in areas of earthquake hazards and volcanic eruptions for monitoring subsurface movements of fluids that may be precursors to a dangerous event. The measurement of the movements can cover a depth region from a few hundred feet to thousands of feet.
A further object of the airborne survey system is it can operate at a fraction of the weight of other airborne survey systems that employ large, heavy and costly transmitters. Also, the new survey system can be used offshore where the above mentioned airborne survey bird replaces a xe2x80x9cfishxe2x80x9d towed by a marine vessel, such as a seismic ship.
The subject magnetotelluric airborne survey system includes an aerodynamic airborne survey bird adapted for being towed behind a helicopter or fixed wing aircraft. The survey bird can include a single magnetometer or include two pair of magnetometers for measuring electromagnetic gradients. The magnetometers are attached to angular motion detectors for compensating for errors caused by angular motion of a line between the magnetometers, when in the presence of strong magnet gradients. The bird also includes the orthogonal axis coils for measuring natural magnetic fields in a frequency range of 3 to 480 Hz Further, a nose boom of the survey bird includes orthogonal electric dipoles for measuring electric fields from 0.01 Hz up to 480 Hz. The airborne survey system is used in conjunction with a ground base station that includes a cesium magnetometer, associated recording equipment and a GPS time reference for synchronization with the airborne data.
These and other objects of the present invention will become apparent to those familiar with ground and airborne geophysical survey systems and airborne survey birds when reviewing the following detailed description, showing novel construction, combination and elements as described herein, and more particularly defined by the claims, it being understood that changes in the embodiments in the disclosed invention are meant to be included as coming within the scope of the claims, except insofar as they may be precluded by the prior art.